Based on the electronic band structure, we have calculated the dispersion of the linear and nonlinear optical susceptibilities for the mixed CuAl(S1−xSex)2 chalcopyrite compounds with x = 0.0, 0.25, 0.5, 0.75, and 1.0. Calculations are performed within the Perdew-Becke-Ernzerhof general gradient approximation. The investigated compounds possess a direct band gap of about 2.2 eV (CuAlS2), 1.9 eV (CuAl(S0.75Se0.25)2), 1.7 eV (CuAl(S0.5Se0.5)2), 1.5 eV (CuAl(S0.25Se0.75)2), and 1.4 eV (CuAlSe2) which tuned to make them optically active for the optoelectronics and photovoltaic applications. These results confirm that substituting S by Se causes significant band gaps' reduction. The optical function's dispersion εxx2(ω) and εzz2(ω) / εxx2(ω) , εyy2(ω) , and εzz2(ω) was calculated and discussed in detail. To demonstrate the effect of substituting S by Se on the complex second-order nonlinear optical susceptibility tensors, we performed detailed calculations for the complex second-order nonlinear optical susceptibility tensors, which show that the neat parents compounds CuAlS2 and CuAlSe2 exhibit ∣∣χ(2)123(−2ω;ω;ω)∣∣ as the dominant component, while the mixed alloys exhibit ∣∣χ(2)111(−2ω;ω;ω)∣∣ as the dominant component. The features of ∣∣χ(2)123(−2ω;ω;ω)∣∣ and ∣∣χ(2)111(−2ω;ω;ω)∣∣ spectra were analyzed on the basis of the absorptive part of the corresponding dielectric function ε2(ω) as a function of both ω/2 and ω .